The invention concerns a magnet system for the production of a static homogeneous magnetic field in an investigation volume of a nuclear spin tomograph (magnetic resonance imaging system) as well as a nuclear spin tomograph with such a magnet system. This type of magnet system is known in the art from US-A-4 701 736.
Nuclear spin tomographs, with their functional performance and imaging procedure capabilities, are described thoroughly in the technical literature (by way of example "Medizinische Physik 1983", He/uml/ thig-Verlag, J. Sche/uml/ tz editor). One of the fundamental components is a magnet system which produces a static, very homogeneous magnetic field in an investigational volume which is as large and as easily accessible as possible. Further requirements are insensitivity to interference from external fields, a small stray field, the avoidance of eddy currents in the event of switching magnetic gradient fields, and last but not least simple and economical production and maintenance as well as low operation costs (electrical current, cooling water etc.).
Since the development of the first tomography systems in the 1970's an entire series of various magnet types have been proposed and to a large extent realized. Naturally, they distinguish themselves from one another to the extent in which they fulfill these specific requirements. Of the three main types of magnets, namely, permanent magnets, resistive electromagnets, and superconducting electromagnets, only resistive and superconducting systems will be considered below since permanent magnets are too heavy and too unstable at the field strength required or desired for many applications. The large majority of the magnet systems utilized today and essentially all superconducting versions consist of a lengthwise extended, largely cylindrical pipe shaped coil, which defines a circular cylindrically shaped inner region (by way of example DE 31 23 493, DE 32 45 945.9). This coil can consist of a plurality of partial sections (by way of example the so-called double Helmholtz configuration, DE 32 45 944.0, EP-A-O 011 335, EP-A-O 033 703 or Nachr. Chem. Tech. Lab. 28 (1980) Nr. 12 Pages 861-865). Although in open air coils of the double Helmholtz type, a sidewards access is in principle possible, the two inner coils of the four coil configuration are so close to one another that it is more practical when the field and patient axis coincide and when access is solely along the solenoid axis. For magnets with cylindrical cryostats and/or iron shielding, this is necessary. In tomographs for whole-body examinations of humans, the free access typically exhibits a diameter of approximately one meter with an entire system length of significantly more than two meters. Thereby, in the most common systems, the patient is located within a long relatively narrow pipe which easily leads to claustrophobia. In addition patient observation by the doctor or patient access for further medical procedures is difficult. The situation is similar for magnet systems of the "window-frame type" (DE 34 18 812.6). In this case, there is the additional problem of the large iron mass which is also associated with the so-called H-magnet (DE 36 16 078.4). With these iron yoke containing pole piece magnets, the free access is significantly better than in the solenoid versions, however there are disadvantages associated with the large weight and with eddy currents induced in the iron of the pole piece and/or the poorly controllable magnetic after-effects in the event of switched gradient fields. An advantage is that the patient axis is perpendicular to the field axis (as is also the case for "window-frames") and, therefore, solenoid coils can be utilized as high frequency senders and for detector coils which, in turn, are significantly more sensitive than saddle coils. For the production of the magnetic field, extremely short air coils (EP-A-160 350) or configurations with which the homogeneity and thereby the investigational region actually lies outside of the magnet structure (DE-A1-31 40 225) are also known in the art. Up to this point however, these have not been made technically feasible and, in practice, have also failed to achieve the specifications required in nuclear magnetic resonance tomography.
The object of the invention is therefore to effect a magnet system for use in NMR tomography with, relative to the size of the investigational volume in which a largely homogeneous magnetic field must be produced, small axial and radial dimensions as well as relatively small overall weight, which allows for possible access to the investigational volume from many differing directions and, moreover, is largely insensitive to eddy current effects which, by way of example, are associated with gradient switching.